1. Field of Invention
This invention relates generally to semiconductor memories and specifically to content addressable memories.
2. Description of Related Art
Packets of data are relayed across the Internet according to an Internet Protocol (IP) addressing scheme. One commonly used IP addressing scheme is known as IPv4. An IPv4 address is a 32-bit binary address segmented into Network and Host address fields. IPv4 addresses are typically grouped into classes depending upon how many bits are in the Network field. For example, Class A, B, and C IPv4 addresses have 8-bit, 16-bit, and 24-bit Network fields, respectively. Due to static field boundaries, the classfull IPv4 addressing scheme generally results in inefficient use of address space.
A classless IP addressing scheme commonly known as Classless Inter-Domain Routing (CIDR) allows for a floating boundary between the Network and Host fields to more efficiently utilize address space. A CIDR address may be expressed as a standard 32-bit IPv4 address followed by a prefix Z, i.e., IPv4/Z, where the prefix Z indicates the number of bits in the Network field (and thus the prefix length of the CIDR address). For instance, a CIDR address of 168.69.48.112/12 has a 12-bit Network field and a 20-bit Host field.
For routing applications, CIDR addresses are typically stored in a searchable table such as a content addressable memory (CAM) device. A CAM device includes an array having a plurality of rows of CAM cells for storing a plurality of CAM words, e.g., CIDR addresses. During compare operations, a comparand word or search key is compared with all the CAM words in the device. For each CAM word that matches the comparand word, a corresponding match line signal is asserted to indicate the match condition. If the comparand word matches more than one of the CAM words, the match line corresponding to each of the matching CAM words is asserted, and a multiple match flag is also asserted to indicate the multiple match condition. The match line signals are typically provided to a priority encoder to determine the index or address of the highest-priority matching (HPM) CAM word.
For routing applications, it may be desirable to compare only the Network field portion of CIDR addresses stored in a CAM array with the search key. In such compare operations, the Host field portion of each CIDR address stored in the CAM array may be masked so that bits in the Host field portion do not participate in the compare operation. The Host field bits may be masked globally or locally.
In a binary CAM device, data entries stored in an array are masked by a global mask. The global mask stores a mask pattern that masks a predetermined number of bits of each entry in the array. Because addresses within a classfull IPv4 addressing scheme (e.g., Class A) each have the same prefix length, and thus the same number of Host field bits are to be masked during compare operations, binary CAM devices are useful for classfull IPv4 addressing schemes where all data entries have the same prefix.
However, binary CAM devices are not well suited for CIDR addressing schemes in which the CAM data entries may have different prefix lengths, and thus require individual masks of different lengths. During compare operations, it is generally desirable to determine which matching CAM entry is the “best match” or “longest prefix match” (LPM), that is, which matching CAM entry has the longest prefix, (i.e., the fewest number of masked bits). Because the global mask of a binary CAM device masks the same bits for each entry, numerous compare operations may be needed to determine the best match. For example, the global mask is initially set to not mask any bits for a first compare operation. If there is no match, the global mask is set to mask one column for a second compare operation, and so on until a match conditions occurs. The first compare operation that results in a match indicates the best match. Performing multiple compare operations per search key to determine the LPM requires considerable time, and therefore may limit performance.
A ternary CAM array includes a local mask word for each row of CAM cells so that each entry in the ternary CAM array may be individually masked according to its prefix. The ability to individually mask each entry eliminates the need for iterative compare operations per search key, thereby making ternary CAM devices well-suited for CIDR addressing schemes. In one scheme for CIDR address processing, the entries are pre-sorted in the ternary array such that entries with the longest prefix are stored at the highest priority locations (e.g., in the lowest CAM index), and entries with the shortest prefix are stored in the lowest priority locations (e.g., the highest CAM index). During a compare operation, a priority encoder generates the index of the highest-priority match, which is also the longest prefix match because of the ordering of the entries according to prefix length. Since the priority encoder determines the highest priority matching location based on predetermined address assignments, the ordering of entries in the ternary CAM array must be maintained in order to generate the correct results. The prioritizing of the word entries is typically performed by a table management hardware and/or software tool. Thus, when a new entry is written to the CAM array, the table management tool must re-order some or all of the entries in order to maintain proper ordering. The re-ordering of entries in the ternary CAM can add significant overhead to the associated router (e.g., delay and additional hardware and software) and may limit performance.
In other schemes for CIDR address processing in a ternary CAM, such as those described in U.S. patent application Ser. No. 09/338,452 entitled METHOD AND APPARATUS FOR DETERMINING A LONGEST PREFIX MATCH IN A CONTENT ADDRESSABLE MEMORY DEVICE and U.S. patent application Ser. No. 09/406,170 entitled METHOD AND APPARATUS FOR PERFORMING PACKET CLASSIFICATION FOR POLICY BASED PACKET ROUTING, entries may be stored in the ternary CAM array in any order (regardless of prefix length). Circuitry included within the ternary array itself or external to the ternary array can be used to resolve the longest prefix match.
Because each row of CAM cells in a ternary CAM array includes an additional row of mask cells to store the local mask, ternary CAM devices are generally not able to store as many unique CIDR addresses as binary CAMs.
It would be desirable to implement CIDR applications using binary CAM cells to achieve higher storage density without having to perform a lengthy sequence of compare operations for each search key, and to be able to add new CAM entries without having to re-order existing entries.